Problem 6. Which group of fractions can have 40 as a common denominator?
$\frac{1}{2}$; $\frac{3}{4}$; $\frac{5}{7}$
$\frac{5}{7}$; $\frac{1}{8}$; $\frac{3}{4}$
$\frac{1}{2}$; $\frac{3}{4}$; $\frac{1}{8}$
$\frac{3}{4}$; $\frac{1}{12}$; $\frac{1}{8}$